The manufacture of semiconductor devices typically includes a lithography process. Lithography typically involves various combinations of material deposition, etching, and chemical treatment.
A portion of a typical lithography process would proceed as follows. A film (e.g., a metal film) layer is deposited on a substrate. A photoresist layer is then deposited on the substrate (i.e., over the film layer). A photoresist is a photosensitive material that hardens when exposed to light. The photoresist may typically be spun onto the substrate and may include solvents to ensure a uniform coating. Such photoresists may be soft baked after deposition to drive off excess solvents. The photoresist is exposed to light in specific places to modify the solubility properties of the film only in the exposed regions. Typically, a mask (i.e., a transparent plate having a printed pattern) and a light source (scanner) are used to illuminate the specified portions of the photoresist layer. Typically, the wafer may then be postexposure baked and may then be developed to remove the exposed portion. Then the photoresist layer is etched using a chemical treatment. The exposed portions of the photoresist may be positive (i.e., rendered more susceptible to chemical etching) or negative (i.e., rendered less susceptible to chemical etching).
The scanner used to illuminate the specified portions of the photoresist layer is a type of camera that forms the desired image on the photoresist layer. The scanner has a number of optical elements in conjunction, used to project the image of the pattern from the mask. The image proceeds from the last optical element of the scanner. The last optical element is therefore in proximity to the wafer with an air gap between the last lens element and the substrate. The index of refraction of the air is different from the index of refraction of the lens, which may typically be quartz (e.g., silicon oxide) or a calcium fluorite crystal. This mismatch in the indices of refraction results in diffraction that limits the minimum size and image quality of the projected image.
One method of reducing mismatched indices between the last lens element and the photoresist, is to place a liquid on the photoresist layer in contact with both the photoresist layer and the last lens element. The liquid is selected to match the index of refraction of the last lens element at the incident wavelength and is, therefore, referred to as an index-matching liquid (IML). By eliminating the air gap and providing a matched index of refraction, the IML helps reduce the diffraction allowing a greater portion of the image information to proceed from the scanner into the photoresist layer, which allows the projection of an image of higher quality. The IML may be selected to provide the best index-matching properties for a given lithographic process. For example, if a 193 nm wavelength light is used to illuminate the photoresist layer, then a silicon-based lens may typically be used and water has a similar refractive index. For other illumination wavelengths, other types of lenses may be used and an alternative IML may be selected.
One major drawback of putting an IML on the photoresist layer is that IML may have a detrimental impact on the photoresist layer. For example, the liquid-contact properties of the photoresist are such that water in contact with the photoresist layer will diffuse into the photoresist layer. Moreover, constituents of the photoresist (e.g., photoactive compounds) will diffuse from the photoresist layer into the water and may thereby degrade the index-matching properties of the water and the imaging performance of the photoresist.
FIG. 1 illustrates the diffusion between an IML and a photoresist of an immersion lithography exposure system in accordance with the prior art. FIG. 1 includes a substrate 101 having a photoresist layer 102 deposited thereon. An IML 103 is disposed between the photoresist layer 102 and a last lens element 104 for illuminating the photoresist. Last lens element 104 is part of a scanner device (not shown). As shown by the arrows in FIG. 1, the IML 103 is diffusing into the photoresist layer 102 and constituents of the photoresist layer 102 are diffusing into the IML.
One known method of addressing the problem of the diffusion of the IML into the photoresist (or the diffusion of photoresist constituents into the IML), is to put a protective coating (topcoat) on the layer of photoresist that acts as a seal or shell to protect the photoresist. Such topcoats may prevent diffusion and may provide other beneficial properties (e.g., anti-reflective coating).
A serious drawback of putting a protective coating on the photoresist is that an additional process operation is required. This adds to production costs and production times, and lowers manufacturing throughput.